The NAVSTAR (Navigation System with Time and Range) Global Positioning System (GPS) is a space-based radio-positioning and time-transfer system. While the system was originally developed primarily for military purposes, it now also contains a "coarse acquisition" (C/A) channel that is available for general civilian use. GPS provides accurate position, velocity, and time (PVT) information for a given object anywhere on the face of the earth, such as a moving mobile terminal in a vehicle. The NAVSTAR GPS includes three major system segments: (i) a space segment, (ii) a control segment, and (iii) a user segment. Briefly, the space segment has twenty four NAVSTAR satellites, each of which broadcasts radio frequency (RF) ranging codes and navigation data messages. Each navigation data message includes such data as satellite clock-bias data, ephemeris data (precise orbital data of the satellite), certain correction data, and satellite almanac data (coarse orbital data on the 24 satellites). The twenty four satellites are arranged in six orbital planes with four satellites in each plane, and the orbital planes are inclined at an angle of 55 degrees relative to the earth's equator. The control segment primarily consists of a master control station currently at Falcon Air Force Base in Colorado, along with monitor stations and ground antennas at various locations around the world. The master control station monitors and manages satellite constellation. The monitor stations passively track GPS satellites in view and collect ranging data for the satellites. This ranging data is transmitted to the master control system where satellite ephemeris and clock parameters are estimated and predicted. Furthermore, the master control system uses the ground antennas to periodically upload the ephemeris and clock data to each satellite for retransmission in the navigation data message. Finally, the user segment comprises GPS receivers, specially designed to receive, decode, and process the GPS satellite signals.
Generally, the satellites transmit ranging signals on two D-band frequencies: Link 1 (L1) at 1575.42 MHz and Link 2 (L2) at 1227.6 MHz. The satellite signals are transmitted using spread-spectrum techniques, employing ranging codes as spreading functions, a 1.023 MHz coarse acquisition code (C/A-code) on L1 and a 10.23 MHz precision code (P-code) on both L1 and L2. The C/A-code consists of a 1023 bit pseudorandom (PRN) code, and a different PRN code is assigned to each GPS satellite, as selected from a set of codes called Gold codes. The Gold codes are designed to minimize the probability that a receiver will mistake one code for another (i.e., minimize cross-correlation). The C/A-code is available for general civilian use, while the P-code is not. In addition, a 50 Hz navigation data message is superimposed on the C/A-code, and contains the data noted above.
In particular, the navigation message has 25 frames of data, each frame having 1,500 bits. Each frame is divided into five subframes of 300 bits each. At the 50 Hz transmission rate, it takes six seconds to receive a subframe, thirty seconds to receive one data frame, and 12.5 minutes to receive all twenty five frames. Subframes 1, 2, and 3 have the same data format for all twenty five frames. This allows the receiver to obtain critical satellite-specific data within thirty seconds. Subframe 1 contains the clock correction for the transmitting satellite, as well as parameters describing the accuracy and health of the broadcast signal. Subframes 2 and 3 contain ephemeris parameters. Finally, subframes 4 and 5 contain data common to all satellites and less critical for a receiver to acquire quickly, namely almanac data and low-precision clock corrections, along with other data.
The ranging codes broadcast by the satellites enable the GPS receiver to measure the transit time of the signals and thereby determine the range between the satellite and the receiver. It should be noted, however, that range measurements inherently contain an error called an offset bias common to all the measurements created by the unsynchronized operation of the satellite and the user's clocks. See U.S. Pat. No. 5,467,282 to Dennis. This user clock error will yield an erroneous range measurement, making it appear that the user is either closer to or farther from each of the satellites than is actually the case. These measurements are therefore more accurately termed pseudoranges. The navigation data messages enable the receiver to calculate the position of each satellite at the time the signals were transmitted.
In general, four GPS satellites must be in clear view of the GPS receiver in order for the receiver to accurately determine its location. The measurements from three GPS satellites allow the GPS receiver to calculate the three unknown parameters representing its three-dimensional position, while the fourth GPS satellite allows the GPS receiver to calculate the user clock error, and therefore determine a more precise time measurement. The GPS receiver compiles this information and determines its position using a series of simultaneous equations.
In addition, when the GPS receiver is first turned on, it must calculate its initial position. This initial determination is known as a "first fix" on location. Typically, the receiver must first determine which satellites are in clear view for tracking. If the receiver is able to immediately determine satellite visibility, the receiver will target a satellite and begin its acquisition process. If there is no almanac or position information already stored in the receiver, then the GPS receiver enters a "search the sky" operation that searches for satellites. Once the satellites are tracked, the receiver begins receiving the necessary data, as described above.
The "time-to-first-fix" (TTFF) represents the time required for a receiver to acquire the satellite signals and navigation data, and to calculate its initial position. If the receiver has no estimate of current time and position and a recent copy of almanac data, then this process generally takes about 12.5 minutes, which is the time necessary to receive a complete navigation data message assuming a 50 Hz transmission rate and receipt of twenty five frames of data, as described above.
A common problem with the conventional GPS is not having four GPS satellites in clear view of the GPS receiver. This commonly arises, for example, in a city setting such as in an urban canyon--i.e., in the shadow of a group of tall buildings--which can block the GPS satellite signals, or indoors in the buildings themselves. In such situations, the GPS receiver is unable to accurately determine its location using GPS.
Therefore, the need arises to find a replacement for the one or more missing GPS satellite signals. One method for accommodating this problem is to provide pseudosatellite signals that are transmitted in the GPS frequency band. They provide much the same information that the typical GPS satellite does, and are utilized by the GPS receiver in much the same fashion as the typical GPS satellite signal. These signals may originate from dedicated stations that are located on the ground at strategic locations, such as at airports. However, pseudosatellite signals are stronger than the GPS satellite signals and therefore, block the GPS signals. Thus, they generally transmit for only ten percent of the time. That is, they transmit periodically, known as burst mode, such as on for ten percent of the time and off for ninety percent of the time.
In addition to drowning out actual GPS satellite signals, the conventional pseudosatellite signal approach has other disadvantages. For one, there is the need to have specialized dedicated stations at strategic locations to transmit this information. This increases the cost of the GPS, and requires the need for obtaining permission from the landowner to set up and operate such dedicated stations. In addition, the user must be located within some specified distance of the station in order to receive the pseudosatellite signal, which is not always the case. Therefore, there is a need for a more efficient, less costly, and reliable alternative for addressing the situation of an inadequate number of GPS satellites being in clear view of the GPS receiver.
In addition, even when four satellites are in view, and the GPS receiver is readily receiving all of the necessary pseudorange data for calculating its position, there are further common errors present that result in erroneous position determinations. These errors include physical errors such as signal path delays through the atmosphere, i.e., propagation signal delay, and satellite clock and ephemeris errors. In addition, for civilian users, the Government introduces errors for national security reasons, generally known as selective availability errors (SA). SA primarily includes ephemeris data error and clock error, and results in an erroneous position determination of approximately 25 to 100 meters.
In order to help reduce the effects of these errors, a differential GPS (DGPS) may be employed. DGPS can achieve accuracies in the order of ten meters. The typical DGPS architecture includes one or more reference stations at precisely known, fixed reference sites, and DGPS receivers. The reference station includes a reference receiver antenna, a differential correction processing system, and data link equipment. As an example, the United States Coast Guard has set up reference stations that broadcast the differential correction data, which is typically used by ships.
There are two primary variations of the differential measuring techniques. One technique is based on ranging-code measurements and the other is based on carrier-phase measurements. In general, the ranging-code differential technique uses the pseudorange measurements of the reference station to calculate pseudorange or position corrections for the user receivers. The reference station calculates the pseudorange corrections for each visible satellite by subtracting the "true" range, determined by surveyed position and the known orbit parameters, from the measured pseudorange. The reference station typically broadcasts the pseudorange corrections in real-time on a low frequency beacon channel, which is received in real-time by the DGPS receiver. Of course, both the DGPS receiver and the reference receivers could alternatively collect and store the necessary data for later processing. The DGPS receiver selects the appropriate correction for each satellite that it is tracking, and subtracts the correction from the pseudorange that it has measured. For example, with the reference station set up by the Coast Guard, the station will broadcast the pseudorange corrections as radio signals. Ships having DGPS receivers receive this radio signal and process it to correct the pseudorange data obtained from the GPS satellites.
The other differential technique is the carrier-phase differential technique, which is typically used in applications requiring high accuracy such as in surveying or for an aircraft landing system. This method measures the difference in phase of the carrier at the reference and mobile unit. The ambiguity in the integer number of cycles is determined by either bringing the antennae of the reference unit and mobile unit close together (less than one wavelength), or by redundant measurements and complex search algorithms to determine the correct solutions.
Furthermore, DGPS may be designed to serve a limited area from a single reference station, which is generally called a local area DGPS (LADGPS). In the alternative, the system may use a network of reference stations and known algorithms to extend the validity of the DGPS technique over a wide area--known as Wide Area GPS, or WADGPS.
The typical DGPS presents certain drawbacks. One drawback is that the DGPS must use its own frequency band, so as not to interfere with that of the stand alone GPS. In addition, the DGPS receiver presents an additional receiver that must operate independent of the GPS receivers in receiving the differential correction data. These problems work in direct tension with the desire to make such systems as small and compact as possible, with as little additional circuit structure as possible, and still be as efficient as possible in terms of utilizing limited frequency.
Another area of interest for the present invention is cellular technology. FIGS. 1 and 2 show a typical cellular network, and its main components. See U.S. Pat. No. 5,546,445 to Dennison et al. The typical cellular network 100 covers a contiguous area that is generally broken down into a series of cells 110. Each cell has a base station 210 that maintains communication with the mobile terminal 220 (e.g., a cellular phone). The base station 210 includes a transmitter and receiver (or transceiver), and an antenna that transmits a wireless signal over a given area. The transmit power of the base station is directly related to the size of the cell, where the greater the transmit power of the base station, the larger the size of the cell.
The overall management of the cellular system is handled by a mobile telecommunications switching office (MTSO) 120. The MTSO provides numerous functions for the cellular system, such as assigning calls to a cell based on availability and signal strength, call statistics, and billing for the cellular network. The MTSO also functions as the interface between the cells and the Public Telephone Switching Network (PTSN) 140 for connection to the local telephone company 230 and long distance toll centers.
In configuring the cellular network, the desired size of the cell depends on the geographic nature of the coverage area and the amount of traffic expected in that area. Each cell uses a group of assigned frequencies or channels. In addition, where traffic becomes too heavy in a given area, the cell may be split into smaller cells by a process known in the art as "cell splitting." This concept is generally illustrated in FIG. 1.
In many instances, a cellular user also wishes to determine their location. The cellular user may carry around a GPS receiver for determining location. An alternative is to have the GPS receiver incorporated into the cellular mobile terminal. See, for example, U.S. Pat. No. 5,043,736 to Darnell et al and U.S. Pat. No. 5,625,668 to Loomis et al. Methods also exist for determining location in a cellular system independent of GPS in order to determine location, such as using the cellular network infrastructure. Two examples for calculating position (though not the only methods) are (i) using Time Of Arrival (TOA) measurements when the time of transmission of the signal from the base stations is known, or (ii) using Time Difference of Arrival (TDOA) measurements when the actual time of transmission is not known, but periodic signals are available, as explained below.
Referring generally to FIG. 3, a typical urban street pattern 300 is shown to illustrate the first method of using TOA measurements. When the time of transmission of the signal from a base station 310 is known, a mobile terminal 320 simply determines when that transmitted signal is received. The difference in time from transmission to receipt, also known as the propagation delay, multiplied by the speed of light, provides a radial distance measurement R between that base station and the mobile terminal. Calculating the distance between the mobile terminal and three different base stations provides an accurate location fix for the mobile terminal, as the intersection of three spheres.
In the second method of utilizing TDOA measurements, while this approach can also be used when the actual time of transmission of the signal from the base stations is available, it may also be used when such time of transmission is not available, but periodic signals are. This may occur with some cellular systems. Some CDMA (Code Division Multiple Access) systems, such as those conforming to the IS-95 standard, do provide transmissions at well defined times.
The periodic signal entails each of the base stations transmitting periodic signals that are synchronized with one another. In that regard, all of the base stations may transmit their periodic signals at the same exact time, or with some specified timing offset between base stations. In this method, the mobile terminal measures the difference in time between the arrival of a signal from one base station with respect to another. This time difference of arrival (TDOA), together with the known locations of the two base stations and the speed of radio signal transmission, defines a hyperbolic surface with the base stations at the foci. The mobile terminal's location is somewhere on this surface. Thus, a single TDOA measurement does not uniquely define the location of the mobile terminal. However, a similar measurement for signals from other pairs of base stations defines additional surfaces. By measuring the TDOA of the signals from three base stations, three surfaces can be determined, the common intersection of which establishes the location of the mobile terminal.
Further information and systems regarding conventional TDOA location systems and methods may be found in Krizman et al., "Wireless Position Location Fundamentals, Implementation Strategies, and Sources of Error," presented at the IEEE Conference on Vehicular Technology, Phoenix, Ariz., May 5-7, 1997 and in the issue of the IEEE Communications Magazine, April 1998, Vol. 36, No. 4, pages 30-59. The entirety of this reference is hereby incorporated into the present disclosure for its teachings regarding conventional TDOA location methods and systems.
However, problems exist with using these two methods for determining location. One significant problem results from multi-path errors. Such errors result from changes in the transmission path of the signal that the mobile terminal receives from the base station. For example, when the user of the mobile terminal goes around a corner, the mobile terminal may receive a new signal from the base station that has followed a completely different transmission path compared to the old signal that the mobile terminal was previously receiving before the user turned the corner. Therefore, the distance traveled by the signal will likely differ. This causes a change in time measurement by the mobile terminal that does not accurately represent the actual distance change of the mobile terminal from the base station, thereby rendering an inaccurate location determination by the mobile terminal.
Another problem encountered is that the typical clock in a cellular mobile terminal does not measure time precisely, and may have a tendency to drift, generally known as clock drift. Therefore, the time measurements made by the terminal are not extremely accurate, which results in an erroneous time--and therefore location--determination. The error due to the drift grows larger the longer the mobile terminal clock is used.
In sum, as shown above, a need exists for a more efficient and less costly structure compared to the conventional DGPS receiver. In addition, a need exists for more efficient, reliable, and effective solutions to address the problem of receiving an inadequate number of satellite signals from the GPS satellites.